Haemolytic Uraemic Syndrome

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Haemolytic uraemic syndrome (HUS) was first described in 1955.[1]It is is a triad of:

Microangiopathic haemolytic anaemia (Coombs' test negative).

Thrombocytopenia.

Acute kidney injury (acute renal failure).

It is the most common cause of acute kidney injury in children and its incidence appears to be growing worldwide. Some children may develop partial or incomplete HUS with thrombocytopenia, with or without anaemia, but the serum creatinine concentration remains normal.[2]

Typical (or infection-induced) HUS is most commonly associated with Escherichia coli with somatic (O) antigen 157 and flagella (H) antigen 7 - hence the designation O157:H7. It produces a toxin called Shiga toxin or verotoxin - hence, alternative names are Shiga toxin-producing E. coli (STEC) or verotoxin-producing E. coli (VTEC). HUS is a systemic disease caused by damage arising from the circulating toxin which binds to endothelial receptors, particularly in the renal, gastrointestinal and central nervous systems. Thrombin and fibrin are deposited in the microvasculature. This occurs early in the disease, prior even to the development of HUS and may be why antibiotics confer no benefit. Erythrocytes are damaged as they pass through partially occluded small vessels and subsequent haemolysis occurs. Platelets are sequestered but without the cascade of clotting factors as in disseminated intravascular coagulation (DIC).[2]

Other pathogens may induce HUS - for example:

Bacteria such as Streptococcus pneumoniae and Shigella dysenteriae type 1.[3]

HUS is a rare disorder with an annual incidence of 6.1 cases per 100,000 children aged under 5 years (compared with an overall incidence of 1 to 2 cases per 100,000).[7]

In the UK and Ireland, the two British Paediatric Surveillance Unit prospective surveys (1985-1988 and 1997-2001) of HUS in children aged under 16 years reported a strong association in the incidence of HUS and age, with children under 5 years of age being most commonly affected.[8]

Over 90% of cases in children are secondary to infection. Approximately 15% of cases of E. coli O157 infection will develop HUS.[8] The development of HUS may occur up to two weeks after the initial onset of symptoms and may develop after apparent recovery from the initial acute illness. Cases may be sporadic or occur as part of larger outbreaks. The largest documented outbreak in England occurred in Cumbria in 1999 and was associated with pasteurised milk. It affected 114 people.

About 10% of HUS cases are atypical and are not caused by Shiga toxin-producing bacteria or streptococci. Patients without evidence of underlying infection should be fully investigated, in particular looking for complement gene mutations.[9]

HUS occurs worldwide but is less widely reported in countries with less developed medical services.

The incubation period for E. coli O157 is 1 to 6 days. HUS usually arises within 14 days after the onset of diarrhoea due to VTEC infection (the mean period reported in the UK is 6-8 days but this interval may be more prolonged).[8] Risk of transmission in a nursery setting may be as high as 38%.

Risk factors

Rural populations >urban populations.

Warmer summer months (June-September).

Young age (6 months to 5 years).

Older people or those with altered immune response.

Contact with farm animals.

Children with E. coli O157 enteritis should not go back to school or nursery until they have had two negative stools.[5]Post-symptomatic shedding can occur but the highest transmissibility is thought to occur during the acute diarrhoeal phase.

Presentation

The classical presenting feature is profuse diarrhoea that turns bloody 1 to 3 days later. It is rare for the diarrhoea to have been bloody from the outset. About 80-90% of children from whom the organism is isolated will develop blood in the stool. It is usually at this stage that they are admitted to hospital.

Most adults infected with E. coli O157 remain asymptomatic.

There is often fever, abdominal pain and vomiting:

About 50% of patients give a history of having had fever but most are afebrile by the time they reach hospital. This is in contrast to most other causes of bacterial colitis.

Abdominal pain is more marked than with other forms of bacterial enteritis and defecation is often painful.

Early clinical signs of HUS may not be specific and it is recommended that if there is any suspicion of HUS (eg, any case of bloody diarrhoea) then assessment should include:

FBC and film: evidence of haemolysis, anaemia and thrombocytopenia. Raised white cell count and low platelet count are early indicators of development of HUS. Features of microangiopathic haemolysis include falling haemoglobin, fragmented red cells on blood film examination, and low or falling platelet count.

Renal function and electrolytes: a rise in urea and creatinine may be due to dehydration but, if associated with haemolysis and thrombocytopenia, indicates the onset of HUS.

LFTs.

Lactate dehydrogenase (LDH): high LDH is an early indicator of HUS.

CRP.

Clotting screen (reduced values may be seen during active HUS).

Stool culture: stool sample should be sent for culture and phage typing of the E. coli. Genes for virulence factors may be confirmed by polymerase chain reaction (PCR). Many patients will no longer be shedding bacteria but the presence of E. coli O157 has considerable public health implications. NB: serum or saliva samples may be used in the absence of a faecal sample when screening contacts during outbreaks..

Urine: urinalysis (haematuria or proteinuria appear early in HUS); urine sent to laboratory for culture.

These investigations should be repeated if any clinical deterioration occurs. Whenever atypical HUS is suspected, further specialist tests are warranted, including:[9]

General management includes appropriate fluid and electrolyte management, antihypertensive therapy and dialysis where required. Circulating volume must be kept up to protect the kidneys; simply replacing losses with crystalloid and keeping up with faecal loss is inadequate, as circulating volume will be lost by vascular leakage. Where kidney failure occurs, indications for dialysis are as for any other cause of acute kidney failure.

Novel therapeutic approaches include the use of monoclonal antibodies which block complement activity.[12] Approaches to atypical HUS include:[7]

Early plasma exchange (removing mutant complement proteins).

Specific targeted treatments - eg, factor H concentrate.

Liver or liver and kidney transplants.

Complications

Gastrointestinal:

Intestinal strictures and perforations.

Intussusception and rectal prolapse.

Pancreatitis.

Severe colitis.

Neurological:

Altered mental state.

Cerebrovascular accident (CVA).

Seizures.

Renal:

Acute kidney injury.

Chronic kidney disease.

Haematuria.

Hypertension.

Proteinuria.

Prognosis

Typical HUS with a diarrhoeal prodrome usually has a good prognosis. The two British Paediatric Surveillance Unit prospective surveys in the UK and Ireland (1985-1988 and 1997-2001) of HUS in children aged under 16 years reported a mortality rate of HUS in the first of these surveys to be 5.6% and this had decreased to 1.8% in the second survey.[8]

Death due to HUS is nearly always associated with severe extrarenal disease, including severe central nervous system involvement.

Fatality is highest in infants, small children and the elderly.

Whilst renal recovery is the norm, there have been permanent and serious renal sequelae (hypertension, proteinuria, diminishing glomerular filtration rate) found in 5-25% of HUS patients.[13]

Atypical HUS often has a poorer prognosis, with death rates reportedly as high as 25% and progression to end-stage kidney disease in 50%.[7]

Prevention

The organism is very common in cattle and a low level of infection causes clinical disease. Prevention is based on reducing faecal contamination during slaughtering and processing.

Good personal hygiene measures - eg, hand-washing before and after food-handling and eating, after toilet use and after contact with farm animals.

Increased public awareness about good food hygiene - eg, cook meat and meat products well, especially where minced or in burger form; avoid cross-contamination between raw and cooked food.

Early diagnosis enables early supportive treatment and better ultimate prognosis. Similarly, early identification of an outbreak enables public health measures to be put in place to prevent further cases. Separating known cases from their younger siblings may also be an appropriate measure.[14]

Conjugate vaccines against E. coli O157 are in development - phase 3 trials are awaited.[15]

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